1. Field of the Invention
This invention relates to a method of processing a radiation image in a radiation image recording system used for medical diagnosis and an apparatus therefor. This invention particularly relates to an image processing method and apparatus in a radiation image recording system in which a stimulable phosphor is used for recording radiation image information as an intermediate recording medium and the image recorded therein is read out for reproduction and finally recorded on a recording medium.
2. Description of the Prior Art
It is known in the art as disclosed in U.S. Pat. No. 3,859,527 to record radiation image information in a stimulable phosphor by exposing the phosphor to radiation transmitting through an object like a human body, then read out the recorded information by use of a photodetector by stimulating the phosphor with a scanning laser beam or the like, and record the read out information on a recording medium by modulating a recording laser beam or the like with the information read out by the photodetector.
The above-described radiation image recording system is far more advantageous than the conventional radiography using a silver halide photographic film in that the image is recorded over a very wide range of radiation exposure. Therefore, this system is particularly valuable when it is applied to medical diagnosis in which the human body is recorded like in the conventional radiography. By using this system for such purposes, it becomes possible to obtain the information which cannot be obtained in conventional radiography because of the limited exposure range that can be recorded and observed on conventional X-ray film.
Further, since the X-rays do harm to the human body, it is impossible from the viewpoint of safety to expose the human body to X-rays of high dosage. Therefore, it is desired that the necessary information in the radiography be obtained by exposing the human body only once to the X-ray of comparatively low dosage. On the other hand, radiographs should preferably have both wide exposure latitude and high image quality of high contrast, high sharpness, low noise etc. for viewing and diagnosis. Unfortunately, since the conventional radiography is designed so as to satisfy all the required conditions to some extent, the range of recording density or the ability to record various levels of information and the image quality are both insufficient and neither of these properties are completely satisfied.
The radiation image recording system as mentioned hereinbefore disclosed in U.S. Pat. No. 3,859,527 does not solve the above problems in the conventional radiography, though the system itself is a novel one.
The inventors of the present invention have found a new process and apparatus which solve the above mentioned problems and satisfy both the wide exposure latitude and the high image quality of high contrast, high sharpness and low noise in a radiation image recording system using a stimulable phosphor as disclosed in a copending application; United States patent application Ser. No. 104,855 now U.S. Pat. No. 4,315,318. According to this method and apparatus, a radiation image having high diagnostic efficiency and accuracy can be economically obtained at a high speed.
The method and apparatus of the above mentioned patent are characterized in that, in the course of scanning a stimulable phosphor with a stimulating ray, reading out the radiation image information recorded thereon, converting the read out information into an electric signal and recording a visible image on a recording medium using the electric signal, an unsharp mask signal Sus corresponding to a super-low frequency is obtained for each scanning point and a signal conversion represented by a formula EQU S'=Sorg+.beta.(Sorg-Sus)
where Sorg is an original image signal read out from the stimulable phosphor and .beta. is an emphasis coefficient is performed to emphasize the frequency component above the super-low frequency. In this method and apparatus, the emphasis coefficient .beta. may be a fixed value or a variable value. In the latter case, the emphasis coefficient .beta. may be varied with either Sorg or Sus.
However, according to the research and investigations conducted further thereafter by the inventors of the present invention, an artifact is liable to appear in the low and high brightness regions when the emphasis coefficient .beta. is fixed. When the emphasis coefficient .beta. is made variable, on the other hand, it was found that, in case that .beta. was a monotonically increasing function (.beta.'.gtoreq.0) for instance, the artifact was prevented from appearing in the low brightness region where Sorg or Sus was small, but it was difficult to prevent the appearance of the artifact in the form of black lines in the muscle area around the boundary of a muscle and a bone for example. In other words, in the above described method, it was difficult to completely prevent from occurring artifacts such as the low brightness portions around the boundaries of edges in an image of bones and muscles becoming saturated white to the fog level of the recording medium or the high brightness portions around the boundaries becoming black lines, or the contour lines appearing around the contour of the barium sulfate contrast medium overemphasized in a radiographic image of a stomach (Magen). Thus, in the above described method it was difficult to sufficiently improve the diagnostic efficiency and accuracy of the finally obtained radiation image and there was a fear of making an error in diagnosis in some cases due to the artifacts.